Sterilization such as by ultra high temperature treatment of milk and other protein containing suspensions creates a product which is microbially stable at room temperature for several months. This preservation technique is especially :used for drinks such as milk.
In general, a sterilization such as an Ultra High Temperature (UHT) treatment, raises the temperature of milk to-over 110° C., or even over 125° C. for a few seconds, followed by rapid cooling. UHT-treated milk that is packaged aseptically results in a “shelf stable” product that does not require refrigeration until opened.
Although such products may be microbiologically stable for a long time, there may be some defects which limit the useful shelf life of these products. One of such defects that have been discussed extensively in the art is UHT gelation (also referred to as age thickening). This defect is specifically observed for UHT treated products or otherwise sterilized products that are heat treated and is not observed for products that have not undergone such high temperature heat treatments. For example products that are only pasteurised do not show this defect.
UHT gelation is an aggregation phenomenon that affects shelf-stable, sterilized protein containing products, such as concentrated milk and UHT milk products and other dairy products. UHT gelation may be linked partly to age thickening or age gelation. After weeks to months storage of these products, there is an increase in viscosity accompanied by visible gelation.
In the context of this invention, gelation describes the phenomenon of increase of viscosity and/or elasticity of the product. Gelation is generally linked to the formation of a space-spanning network and therein differs from other phenomena such as sedimentation which is more about a spatial redistribution of ingredients. An example of sedimentation is the heaviest ingredients in a product sinking to the bottom of the packaging.
In the above definition of gelation we do not exclude the possibility of the space spanning network to redistribute after it has formed a space spanning network, i.e. the gel may be subject to syneresis, thereby pushing out some of the liquid trapped by the network. Upon ageing within the maximum shelf life of the UHT product (typically 4 months), the network may have shrunk to a fraction of its original size (>75%), while retaining its elastic properties. Upon severe ageing, which only occurs beyond the maximum shelf life of the UHT product, the network may have shrunk to an even smaller fraction of its original size (>25%).
Note that the ageing process of a product suffering from sedimentation is different from above definition; sedimentation is accompanied by the growth of a layer of ingredients at the bottom of the product. The product does not pass a stage of spanning the whole space. Also, it is well known in the art to use gelling ingredients like κ-carrageenan in liquid UHT products to protect them against sedimentation (usually resulting from heat coagulation). Hence, gelation typically reduces the rate of sedimentation.
In the process of UHT or age gelation, the formation of a space-spanning network may be caused by aggregation of proteins leading to formation of a three-dimensional network. The exact mechanism underlying UHT and/or age gelation is not yet fully understood but it was found that it may be at least partially caused by the proteolytic breakdown of proteins such as the casein in dairy products. It has been described in the art that bacterial or native plasmin enzymes that are resistant to heat treatment may induce the formation of a gel. Another factor that is often cited in relation to UHT and/or age gelation is temperature of storage of the sterilized suspension. Also chemical reactions may be responsible for UHT/age gelation.
WO-A-00/64267 discloses that the major problem for shelf stable milk is age gelation. This document suggests that use of food-grade polyphosphates having at least six phophate groups, such as sodium hexametaphosphate in calcium-fortified milk and dairy-based products prevents age gelation of UHT treated milk.
Although the above-cited documents may offer some improvement to the stability of UHT treated products, further improvement and alternatives thereto are desired.
Furthermore there are documents addressing the sedimentation-stability and precipitation reduction in suspensions. E.g. EP-A-1,197,153 and WO-A-02/065859 address the reduction of precipitation in emulsions or suspensions. These documents do not disclose UHT treated products and the problems related thereto. Also these documents do not provide a solution for the problem of UHT gelation.
The stability of homogeneous suspensions is also addressed in EP-A-1059851. This document teaches that a stable suspension can be made without emulsifiers by the inclusion of a thickener. When these products are UHT treated, they will still show UHT gelation.
Therefore it is an object of the invention to provide a shelf stable sterilized protein containing suspension which uses alternative means of stabilisation.